Arizona Cardinals defensive end Calais Campbell has been fined $18,000 by the NFL for his low hit on Carolina Panthers quarterback Cam Newton, USA TODAY Sports has learned.

Campbell is appealing the sanction, according to a person with knowledge of the situation who did not wish to be identified due to the sensitivity of the case.

Newton was incensed by Campbell’s hit and other shots that he deemed excessive during the Panthers’ 30-20 victory on Sunday, which he contends represent a pattern of blows that have not drawn flags throughout his career.

Campbell's hit below the knee – which Newton said could have seriously injured his knee – was undoubtedly the tipping point that prompted Newton to speak out on the issue and take the matter to NFL Commissioner Roger Goodell.

Newton, who said after Sunday’s game that he doesn’t feel safe on the field, told reporters on Wednesday in Carolina that he had a “great discussion” with Goodell.

Campbell’s hit on Newton did not draw a penalty, and it does not appear that Campbell was pushed into the quarterback by a blocker. According to NFL rules, "no defensive player who has an unrestricted path to the quarterback may hit him flagrantly in the area of the knee(s) or below when approaching in any direction." NFL senior vice president of officiating Dean Blandino said Tuesday the play should have drawn a flag.

It’s plausible, though, that Campbell – who apologized to Newton on the field on Sunday – simply lost his balance.